Formulations based on water soluble gold compounds suitable for coloring ceramic manufactured articles

ABSTRACT

Composition whereby ceramic manufactured articles are colored in shades from pink to violet, such composition includes of a monovalent gold thiolate water solution or mixture of water with a water soluble organic solvent according to the following formulas Au—S—R—X and Au—S—R—H, where R stands for a linear or branched bivalent radical of aliphatic or aromatic or cycloaliphatic or heterocyclic type optionally with substituents, such as for example aminic, amidic, hydroxylic, carboxylic, hydrocarbylic or carbonylic groups or CONH—, in the chain; X stands for a monovalent group selected out of —COOH, SO 2 OH, —OH, —CONH 2 , —NH 2 ; O—P(O)(OH) 2 , in which H atoms may be replaced by alkyl groups and wherein acid group may be salified with amines or alkaline or alkaline earth metals and basic groups may optionally be salified with organic acids

FIELD OF THE INVENTION

The present invention relates to compositions suitable for colouring ceramic manufactured articles and to the relevant colouring process.

In particular, the compositions of the invention consist of water solutions or solutions of water and water-miscible solvents, of gold organic complexes compatible with other colouring cations optionally present in the solution releasing low quantities of corrosive vapours during the firing cycle.

Said solutions allow the obtainment of ceramic manufactured articles in shades from pink to purple to violet after a firing cycle ranging from 750° C. to 1,300° C.

STATE OF THE ART

The use of coloured ceramic manufactured articles as well as the compositions and process adopted to obtain the relevant colours have been known since long. One of the methods most commonly used consists in the addition of powdered pigments, in particular inorganic oxides and mineral colouring matters, to the ceramic mixture (vitrified stoneware) before firing. The ceramic manufactured article is thus coloured through its whole thickness, although with large consumption of colouring matter, which is the most expensive component.

According to a process used, the surface of the ceramic material is caused to absorb, either after partial firing (as disclosed e.g. in German patent 2,012,304) or simply after moulding and before firing (as disclosed e.g. in Swiss patent 575,894), a water solution of inorganic salts or metal complexes (as disclosed e.g. in Sprechsal, vol. 119, No. 10, 1986, in EP 0704411 and in patent PCT, WO 97/38952), which become stable colours at high temperature during the ceramic firing cycle.

The water solution is applied to the ceramic material before final firing. This process is particularly advantageous because it allows the colouring of very thin layers: therefore, it is widely used for flat manufactured articles (such as e.g. floor and wall tiles).

Another problem to be solved when using colours in a water solution is the obtainable depth of colour penetration into the ceramic material. In fact, it was experimentally found that the depth of penetration depends on several parameters, such as the viscosity and surface tension of the colouring solution, the application temperature, the quantity of water optionally sprayed on the manufactured article once the colouring solution has been applied. The water solution is applied to the ceramic manufactured article by immersion, spraying, disk, and silk-screen techniques.

Of cardinal importance is the application technique: in particular, the quantity of colouring solution that may be applied by disk and spraying techniques is as high as 400 to 600 g/m2; by silk-screen type techniques it usually amounts to 100 to 200 g/m2 and sometimes even to 400 g/m2, when thickened screens made of a small number of threads are used.

Silk-screen type techniques are very much in demand, being the only techniques allowing graphic decorations and drawings, which otherwise cannot be obtained, and requiring lower quantities of colouring matter. When said techniques are used, the colouring solutions are to be thickened with appropriate thickening agents, e.g. modified glucomannans, starch and modified starch derivatives, cellulose and modified cellulose derivatives, or other polymeric substances, soluble or dispersible in a water solution.

Colour penetration into the ceramic material before firing can be obtained by spraying relatively high quantities of water on the manufactured article after application of the colouring solution. However, the resulting colours are less intense than those obtained using other techniques.

Colour penetration into the material is particularly important in the case of “smoothed” vitrified stoneware tiles.

The term “smoothed” means that the vitrified stoneware surface has been abraded with diamond wheels by 0.8 to 1.5 mm and subsequently smoothed or polished with appropriate felt until obtaining a glassy surface.

It follows that colour penetration into stoneware articles to be smoothed after firing must reach a depth of 1.6 mm min.

Other methods have been developed for the smoothing of very thin (1 to 10 0) surface layers of the manufactured article.

PRIOR ART

The use of water solutions of gold compounds for the colouring also in-depth of ceramic manufactured articles is well known. The compounds disclosed in German patent 4,320,072 for said application substantially are gold chlorides also reported in the preceding literature (cf. “Encyclopedia der technischen Chemie”, F. Ullmann, 1929, vol. 4, pp. 837-838). However, the gold chloride solution suffers from the inconvenience of being strongly acid, as it contains hydrochloric acid. In the absence of excess hydrochloric acid, the solution is unstable and the gold compound hydrolyses easily with formation of insoluble compounds.

It follows that the solution is corrosive and impairs the apparatus used. In particular, in the case of silk-screen type technique, it rapidly impairs the printing screen.

WO 97/21646 discloses the use of gold sodium thiosulphate solutions, Na₃Au(S₂O₃)₂, stabilised with sodium sulphite, for ceramic surfaces colouring by water solution absorption.

From the compositions of solutions 1, 2, and 3 described therein, it is possible to calculate that 4.7 g SO₂ or 5.88 g SO₃, or an intermediate value in the case of mixtures thereof is released per g Au deposited on the surface.

In both cases (use of gold chloride and gold thiosulphate), high amounts of strongly corrosive vapours rapidly impairing the heater metal structures, are released. Therefore, vapours are to be abated to prevent the emission of same into the environment.

The use of precious metals water solutions in high concentrations, in the form of thiol derivatives, to obtain thin metal films for decorative purposes, e.g. for dishes, or for electronic purposes, e.g. for printed circuits, is already known.

The following are examples of said use:

1. U.S. Pat. No. 5,545,452 discloses the use of Au thiolates water solutions with a metal content of 2 to 25% by wt. (column 3, rows 10 to 20) to obtain thin metal films for decorative purposes (column 1, rows 38 to 40);

2. EP No. 514,073, like the U.S. patent above, discloses the use of thiolates water solutions to obtain thin films made of Au or other precious metals to decorate the outer surface, and not the inside, of baked ceramic manufactured articles.

Technical Problem

Considering that it is very simple to colour ceramic materials by disk, spraying and silk-screen techniques, the ceramic industry is highly interested in the possibility of using colouring water solutions based on gold to be applied by said techniques, and offering the advantage of

releasing the lowest possible quantity of very noxious or corrosive vapours in the heater;

being compatible with water solutions of organic derivatives of other cations used for the superficial and in-depth colouring of ceramic manufactured articles.

Therefore, it is an object of the present invention to provide colouring formulations in the form of water solutions, which

release low quantities of corrosive vapours (≦2 g SO₂/g Au deposited);

are compatible with water solutions of organic derivatives of other cations used for the superficial and in-depth colouring of ceramic manufactured articles;

colour the ceramic manufactured articles at their surface and to a depth of at least 1 mm.

The Applicant, who has full-fledged experience in the production and sale of colouring matters for ceramic tiles, has now found that water solutions or water mixtures with alcohols or other water-miscible organic solvents, of monovalent gold organic derivatives, can be used to obtain-after firing-colour shades varying from pink to purple to violet on manufactured articles consisting of a conventional ceramic mixture.

The water or hydroalcoholic solutions being an object of the present invention are particularly useful for colouring tiles of vitrified stoneware, either at their surface or to a depth of 1 to 3 mm from their surface.

In-depth colouring is essential for maintaining the manufactured article decoration after smoothing.

It is, therefore, a fundamental feature of the present invention to use water solutions or water mixtures with hydrophilic organic solvents, of gold organic complexes, which are compatible with derivatives of other cations and release low quantities of corrosive vapours (≦2 g SO₂/g Au deposited).

Said solutions are used to treat ceramic articles before firing. This makes it possible, after firing, to obtain pink, purple and violet coloured articles, when solutions contain gold only, or new colour shades when solutions are mixed with derivatives of one or several of the following cations: Fe, Cr, Co, Mn, Cu, Ru, Pd, Zr, V, Ni, Sb, W, Zn, Sn.

The gold compounds that may be used according to the present invention belong to the class of monovalent gold thiolates, wherein Au is bound to an S atom, and are substantially represented by the following general formulas:

Au—S—R—X and Au—S—R—H

where R stands for a linear or branched bivalent radical of aliphatic or aromatic or cycloaliphatic or heterocyclic type, optionally with substituents, such as for example aminic, amidic, hydroxylic, carboxylic, hydrocarbylic or carbonylic groups or CONH—, in the chain;

X stands for a monovalent group selected out of —COOH, SO₂OH, —OH, —CONH₂, —NH₂; —O—P(O)(OH)₂, in which H atoms may be replaced by alkyl groups and wherein acid groups may be salified with amines or alkaline or alkaline earth metals and basic groups may optionally be salified with organic acids.

In particular, monovalent Au thiolates to be used according to the present. invention may be in the form of alkaline, alkaline earth metal salts or of variously substituted amines.

The water solutions of Au compounds according to the invention are stable, substantially neutral and release a quantity of corrosive vapours of 2 g SO₂ max./g Au deposited on the manufactured article surface to be coloured.

Said solutions are used in quantities corresponding to 0.1 to 20 g Au (as element) per m2 of surface to be coloured.

By way of example, the gold thiolates that may be used according to the invention derive from:

(N) acetylcysteine:

4-mercaptopyridine:

2-mercaptoacetyl-glycine:H—S—CH₂—CO—NH—CH₂COOH

2-mercaptopropionyl-glycine:

(d,I) mercaptosuccinic acid:

4,6-dihydroxy-2-mercapto-pyrimidine:

2-mercapto benzimidazole:

cysteine:

2-mercaptopropionic acid:

The compatibility of the gold thiolates according to the invention with the ceramic colouring systems based on metallic compounds (in particular of Fe, Ni, Cr, Co, Sn, Mn, Cu, Ru, Pd, Zr, V, Sb, W, Zn, Sn) in aqueous solutions has been ascertained through stability tests of the aqueous solutions containing beside Au thiolate also one or more compounds of the above mentioned metals and also through ceramic coloration tests by the same aqueous solutions after ageing. Unforeseable very valuable colours were obtained.

Gold thiolates to be used according to the invention may generally be prepared by causing an Au(III), in the form of a tetrachloroauric acid, to react in water solution with a thioether S(R′)₂, giving the reduction of Au(III) to Au(I), and then with the desired thiol HSR″ (R″=—RX or —RH as per the general formulas shown above):

This method, described in Inorganic Synthesis, 23 (1985) pp. 191-195, is based on some reactions reported therein.

A typical process for applying colouring compositions according to the invention consists in the following steps:

a) drying at 100° C. of the article to be coloured to a water residue of 0.5% by wt. max.;

a.1) pre-treatment, if any, of the dried article with water up to a max. quantity of 300 g/m2 manufactured article;

b) treatment of the pre-treated article with a water solution of the colouring composition in a quantity of 30 to 600 g/m2 of the final coloured surface;

b.1) post-treatment, if any, of the treated article with water up to a max. quantity of absorbed water of 300 g/m2 ceramic manufactured article;

c) equalisation of the post-treated article at room temperature for 8 hours to homogenise the solution absorption;

d) oven firing according to the usual ceramic cycle at a temperature of 1,000 to 1,300° C.

The concentration of Au thiolate solution to be used according to the invention generally ranges from 0.1 to 2% Au (expressed as element).

Some examples of the colours obtained by experimental runs are shown in the Table 1 reported hereinafter.

All runs were carried out on the basis of the following process:

1) drying at 1000° C. of two 33×33 cm supports made of mixture A to a water residue of 0.5% by wt. max.;

2) supports cooling to room temperature;

3) deposition of 0.4 g of each solution on 10 cm2 of surface of each support;

4) supports equalisation at room temperature for 2 hrs and for additional 2 hrs in a thermoventilated oven at 600° C. to homogenise the solutions absorption;

5) oven firing according to the usual ceramic cycle;

6) removal of a surface layer (0.8-0.9 mm) and smoothing;

7) colour detection on non-smoothed and smoothed support;

8) non-smoothed support cutting and penetration depth detection.

TABLE 1 Pentra- Ex. Complexing agent Colour before Colour after tion n° % element smoothing smoothing (mm) 1 D,L mercaptosuccinic Parma red Pink 1.2 acid, 0.4% gold 2 acetylcysteine, 0.4% gold Pink Light pink 2 3 thiolactic acid, 0.4% gold Pink Light pink 2 4 cysteine, 0.4% gold Parma red Pink 1.4

The composition of the ceramic mixture used is as follows (% by wt.) SiO₂ 64.4%; Al₂O₃ 21.8%; K₂O 3.8%; Na₂O 0.8%; CaO 0.6%; MgO 0.1%; TiO₂ 0.3%; Fe2O₃ 0.2%; ZrSiO₄ 5%; H₂O to 100%.

Colours are as per the Colour Atlas.

EXAMPLE 5 Compatibility of Some Au Thiolates With Other Cations

Compatibility tests were carried out with the following water solutions: Au acetylcysteinate prepared with the method disclosed in the following at page 11, line 6 and diluted up to 0.4% Au, Au mercaptosuccinate (0.4% Au), cobalt ammonium citrate (2% Co), Ni ammonium citrate (2% Ni); chromium ammonium citrate (2% Cr), iron ammonium citrate (Fe 2%), tin glycolate (Sn 2%).

The mixtures consist of Au derivative solution (50%) and of another cation solution (50%).

The results obtained, after the days reported below, are as follows:

Au acetylcysteinate Au mercaptosuccinate Co 30 days, clear solution 30 days, clear solution 60 days, clear solution 60 days, clear solution colour turned from — reddish violet to wine red Ni 30 days, clear solution 30 days, clear solution 60 days, clear solution 60 days, clear solution Cr 30 days, clear solution 30 days, clear solution 60 days, slightly opaque sol. 60 days, slightly opaque sol. Fe 30 days, clear solution 30 days, clear solution 60 days, clear solution 60 days, clear solution Sn 30 days, clear solution 30 days, clear solution 60 days, clear solution 60 days, clear solution

EXAMPLE 6

The compatibility of some Au thiolate according to the invention with other colouring cations has been tested in comparison with two Au compounds of the prior art, namely NaAuCl₄ and gold-sodium thiosulphate Na₃Au(S₂O₃)₂. The obtained results are reported in Tables 3 and 4.

The tested products in form of aqueous solutions containing 1% by w. Au are prepared as follows.

Product (1) Au Thiolacetate (Au-TL/3)

30 g aqueous solution of tetrachloroauric acid (corresponding to 1 g Au) is added with aqueous solution of NaOH up to a pH of 8 (solution A). Thiolactic acid 1.65 g is dissolved in 30 g H₂O and added with NH₄OH aqueous solution 30% by w. up to a pH=7 (solution B). The solution B is added to the solution A, then the mixture is added with NH₄OH up to pH 9 and with H₂O up to a total weight of 100 g.

Product (2) Au Sodium Thiosulphate (Au-Bk.Giulini)

Comparison Test.

In 91.6 g of H₂O are dissolved 1.7 g Na₃Au(S₂O₃)₂ and 6.7 g of Na sulphite (molar ratio Au/Na₂SO₃=0.9/10) as disclosed by WO97/21646 in Table 3. This ratio is the best as it regards the stabilization of the solution as declared by the Applicant: no precipitate occurs in the solution when it comes in contact with a piece of metal.

Product (3) Au Acetylcysteine (Au/CST)

30 g of aqueous solution of tetrachloroauric acid (corresponding to 1 g Au) are added with NaOH aqueous solution up to a pH of 8 (solution A). N-acetylcysteine 5.1 g in 30 g H₂O are added with NH₄OH aqueous solution 30%, up to a pH of 7 (solution B). The solution B is added to the solution A, then the mixture is added with NH₄OH up to pH 9.5 and with H₂O up to a total amount of 100 g.

Product (4) Au Acetylcysteine (Au-CST/9)

30 g of aqueous solution of tetrachloroauric acid (≈1 g Au) are added with NaOH aqueous solution up to a pH 8 (solution A). N-acetylcysteine 5.1 g in 30 g H₂O are added with NaOH aqueous solution up to a pH 7 (solution B). The solution B is added to the solution A, then the mixture is added with NaOH up to pH 9 and with H₂O up to a total amount of 100 g.

Product (5) Au Acetylcysteine (Au-CST/2)

30 g of aqueous solution of tetrachloroauric acid (1 g Au) are added with NaOH aqueous solution up to a pH of 8 (solution A). N-acetylcysteine 2.55 g in 30 g H₂O are added with NH₄OH 30% aqueous solution up to a pH 7 (solution B). The solution B is added to the solution A, then the mixture is added with NH₄OH up to a pH 9.5 and with H₂O up to a total amount of 100 g.

Product (6) NaAuCl₄ (Au—Cl)

Comparison Test.

30 g of aqeuous solution of tetrachloroauric acid (≈1 g Au) are added with NaOH aqueous solution up to a pH of 2.5 and with H₂O up to a total amount of 100 g.

Product (7) Au Acetylcysteine (Au-CST/10)

30 g of aqueous solution of tetrachloroauric acid (≈1 g Au) are added with NaOH aqueous solution up to a pH of 8 (solution A). N-acetylcysteine 2.55 g in 30 g H₂O are added with NaOH aqueous solution up to a pH 7 (solution B). The solution B is added to the solution A, then the mixture is added with NaOH up to a pH of 9.5 and with a H₂O up to a total amount of 100 g.

In the following Table 2 are reported the tested products with the neutralizing agent used (NaOH or NH₄OH) and the molar ratio between Au and SO₂ (or SO₃) developed during the ceramic firing of the treated manufactured articles.

The Au acetylcysteine solutions products 3 and 4 have been prepared using an excess of acetylcysteine and consequently these solutions show a content of S higher then in the products 5 and 7. The use of an excess of acetylcysteine affords an higher stability of the Au-acetylcysteine solution.

TABLE 2 Product ref Neutralizing n° Au compound agent Au/SO₂ 1 Au-TL/3 Au thiolactate NH₄OH+NaOH 1:1   2 Au-Bk.Giulini Na-Au thiosulphate — 1:4.7 stabil. Na₂SO₃ 3 Au/CST Au acetylcysteine NH₄OH+NaOH 1:2 4 Au-CST/9 ″ NaOH 1:2 5 Au-CST/2 ″ NH₄OH+NaOH 1:1 6 Au-Cl NaAuCl₄ — — 7 Au-CST/10 Au acetylcysteine NaOH 1:1

The tests reported in the following tables 3 and 4 have been carried out with solutions consisting of mixtures of Au derivative solution (50%) and of other cations solutions (50%)

TABLE 3 Comparative tests for Au-acetylcysterine and Au-thiolactate, reference product Na—Au thiosulphate (Au-Bk.Giulini) Aqueous sol. % cations and Au-CST Au-CST/9 pH Au-TL/3 (1) Au-Bk.Giulini (2) (3) (4) Co amm. 60 d: OK 9 d: little XXX on the 60 d: OK 60 d: OK Citrate Co = 8% bottom pH 7.5-8.5 16 d: many XXX of both Au and Co, on the bottom and on the wall Co amm. 60 d: OK 23 d: some very little 60 d: OK 60 d: OK Citrate Co = 4% XXX pH 7.5-8.5 30 d: many XXX (ocher colour) 60 d: many XXX of Au and Co on the bottom and on the wall NaSb tartrate 60 d: OK 8 d: little greyish 60 d: OK 60 d: OK Cr acetate sediment on the Sb = 12.8% bottom Cr = 1.8% 15 d: little greyish pH 4.5-5 sediment on the bottom and on the wall 22 d: Au sed. on the bottom and on the wall NaSb tartrate 60 d: OK 60 d: little dark sed. 60 d: OK 60 d: OK Cr acetate on the bottom Sb = 6.4% Cr = 0.9% pH 4.5-5 Cr amm. 60 d: OK 60 d: OK 60 d: OK 60 d: OK Citrate Cr = 6.8% pH 7-8 Cr amm. 60 d: OK 60 d: OK 60 d: OK 60 d: OK Citrate Cr = 3.4% pH 7-8 Ni amm. 60 d: OK 60 d: OK 60 d: OK 60 d: OK Citrate (transparent (transparent (dark sol.) (dark sol.) Ni = 7.6% greenish greenish sol.) pH 7-8 sol.) Ni amm. 60 d: OK 60 d: OK 60 d: OK 60 d: OK Citrate (transparent (transparent (dark sol.) (transparent Ni = 3.8% greenish greenish sol.) green sol.) pH 7-8 sol.) Cu amm. 60 d: OK 60 d: little sediment 60 d: OK 60 d: OK Citrate Cu = 8% on the bottom pH 7.5-8.5 Fe amm. 60 d: OK 60 d: OK 60 d: OK 60 d: OK Citrate Fe = 9.8% pH 4.5-5 Fe amm. 60 d: very 22 d: very light, black 60 d: OK 60 d: OK Citrate Fe = 4.9 light, black dust = 29 d pH 4.5-5 dust 60 d: very light, black dust Fe CTR/13 60 d: OK 8 d: sediment on the 60 d: OK 60 d: OK (see foot note) bottom Sn glycolate 60 d: OK 60 d: very light black 60 d: OK 60 d: sol. is Sn = 7.6% sediment on the gelled on all pH 4.5-5 bottom the wall and on the bottom of the test tube, but it is yet liquid and transparent Mn amm. 60 d: OK 8 d: white precipitate 60 d: OK 60 d: OK Citrate on the bottom and Mn = 4.5% on the walls pH 7.8-8.8 Sodium 60 d: OK 8 d: very light white 15 d: some 60 d: OK metavandate sediment very little V = 6.8% (= 15 d = 22 d) 29 d: black mote pH 9.5-10 bright, light sediment 22 d: some very little black mote 29 d: some black extended XXX (little rods) = 60 d V glycolate 60 d: OK 29 d: very light black 60 d: OK 60 d: OK V = 8% dust pH 5-5.5 60d: ppt Au and V Ru glycolate 28 d: ppt of 7 d: remarkable 60 d: OK 7 d: light Ru = 6% Au on the black sediment black pH 2-2.5 wall 14 d: remarkable precipitate 60 d: wall black sediment on 14 d: very and bottom the bottom and on light black of the test the wall precipitate = tube coated 60 d by Au Ru glycolate 28 d: very 7 d: remarkable 60 d: OK 7 d: very Ru = 3% light coat on black precipitate. On light black pH 2-2.5 the wall the wall noticeable precipitate 60 d: Au on sediment of Au 14 d: very the wall and light black on the precipitate = bottom 60 d

d=days

XXX=crystals

ppt=precipitate

sol.=solution

Fe citr/13=10 g hydrate iron citrate Aldrich (Fe 18÷19%) in 20 g H₂ O are added with 10 g of NH₄OH 30%, find pH=7; Fe=4.5÷4.75% by w.

TABLE 4 Comparative tests for Au-acetylcysterine: reference product Na Au Cl₄ (Au—Cl) Aqueous Sol. % cations and pH Au-CST/2 (5) Au—Cl (6) Au-CST/10 (7) Co amm. Citrate 60 d: OK 9 d: very light sed. 60 d: OK Co = 8% pH 7.5-8.5 sol. of violet colour 16 d: as 9 d 23 d: light sed. = 30 d = 60 d Co amm. Citrate 60 d: OK 9 d: OK sol. of violet 60 d: OK Co = 4 pH 7.5-8.5 colour 16 d: OK (violet sol.) 23 d: very light sed. 30 d: light sed. Au = 60 d NaSb tartrate Cr 60 d: OK 8 d: black ppt on the 60 d: OK acetate bottom Sb = 12.8% Cr = 0.9% 15 d: black ppt on pH 4.5-5 the bottom and very weak on the wall = 60 d NaSb tartrate Cr 60 d: OK 8 d: black ppt on the 60 d: OK acetate bottom Sb = 6.4% 15 d: black ppt on Cr = 0.9% the bottom and very pH 4.5-5 light on the wall = 60 d Cr amm. Citrate 60 d: OK 8 d: light brownish 60 d: OK Cr = 6.8% sed. (from the colour pH 7-8 it seems Au) 15 d: brownish sed. Cr amm. Citrate 60 d: OK 8 d: light brownish 60 d: OK Cr = 3.4% sed. (from the colour pH 7-8 it seems Au) 15 d: light brownish sed 22 d: brownish sed Ni amm citrate 60 d: OK (green 60 d: OK (green 60 d: OK (greenish Ni = 7.6% transparent sol.) transparent sol.) transparent sol.) pH 7-8 Ni amm. citrate 60 d: OK (green 60 d: OK (green 60 d: OK (greenish Ni = 3.8% transparent sol.) transparent sol.) transparent sol.) pH 7-8 Cu amm. citrate 60 d: OK 60 d: OK 60 d: OK Cu = 8% pH 7.5-8.5 Fe amm. citrate 60 d: OK 8 d: ppt Au on the 60 d: OK Fe = 9.8% bottom and on the pH 4.5-5 wall Fe amm. citrate 60 d: OK 8 d: ppt Au on the 60 d: OK Fe = 4.9% bottom and on the pH 4.5-5 wall Fe-CTR/13 60 d: OK 8 d: sed. on the 60 d: OK bottom (Au) Sn glycolate 60 d: OK 7 d: ppt Au 60 d: OK Sn = 7.6% pH 4.5-5 Mn amm. citrate 60 d: OK 8 d: very light 60 d: OK Mn = 4.5% brownish sed. (= 15 d = 22 d = 29 g) 60 d: light black sed.) Sodium metavandate 60 d: OK 60 d: very thin black 60 d: OK V = 6.8% film on the wall pH 9.5-10 V glycolate 60 d: OK 8 d: brownish sed. 22 d: very light black V = 8% on the bottom sed. = 60 d pH 5-5.5 60 d: many blue XXX ppt Ru glycolate Ru = 6% 14 d: very light black 7 d: brownish ppt on 7 d: black ppt pH 2-2.5 sed. = 21 d = 28 d 60 d: the bottom and weak black ppt and sed sed. on the wall on the wall Ru glycolate Ru = 3% 60 d: OK 7 d: very light 7 d: black ppt pH 2-2.5 greenish ppt. (= 14 d = 21 d = 28 d) 60 d: greenish sed.

d=days

XXX=crystals

ppt=precipitate

sol.=solution 

What is claimed is:
 1. Composition for colouring ceramic manufactured articles at their surface and to a depth of at least 1 mm in shades from pink to purple to violet, said composition consisting of a solution in water or in mixture of water with a water soluble organic solvent of a monovalent gold thiolate that during the ceramic firing cycle releases corrosive vapours in quantity not higher than 2gSO₂/g Au deposited, comprised in the formulas Au—S—R—X and Au—S—R—H where R stands for a linear or branched bivalent radical of aliphatic or aromatic or cycloaliphatic or heterocyclic class, optionally with substituents, selected from the group consisting of aminic, amidic, hydroxylic, carboxylic, hydrocarbylico and carbonylic groups or CONH—, in the chain; X stands for a monovalent group selected from, the group consisting of —COOH, SO₂OH, —OH, —CONH₂, —NH₂ and —O—P(O) (OH)₂, in which H atoms may be replaced by alkyl groups and wherein acid groups may optionally be salified with amines or alkaline or alkaline earth metals and basic groups may optionally be salified with organic acids.
 2. The composition as claimed in claim 1, wherein the concentration of gold thiolate in the solution corresponds to a content of 0.1% to 2% of Au, expressed as elemental Au.
 3. Composition as claimed in claim 1 wherein the release of corrosive vapours is not higher than 1 gSO₂/g Au.
 4. The composition as claimed in claim 1, wherein Au thiolate is selected from the group consisting of (N) acetyl-cysteine; 4-mercaptopyridine; 2-mercaptopyridine; 2-mercaptoacetyl-glycine; mercaptopropionyl-glycine; 3-mercaptopropionyl-glycine; (d,l) mercaptosuccinic acid; cysteine and 2-mercapto-propionic acid.
 5. The composition as claimed in claim 1 containing, in addition to Au thiolate, other metal compounds having colouring properties for the ceramic material which are selected from the group consisting of organic or inorganic derivatives of the following elements: Co, Cr, Ni, Ru, Mn, Sb, W, Cu, Fe, Zr, V, Zn, Pd and Sn.
 6. The composition as claimed in claim 1, containing in addition to an Au thiolate selected from Au thiolactate and/or Au acetylcysteinate and/or Au mercaptosuccinate, additional metal compounds having colouring properties for the ceramic material which are selected from the group consisting of Cobalt ammonium citrate, Chromium acetate, Chromium ammonium citrate, Nickel ammonium citrate, Ru glycolate, Ru ammonium citrate, Mn ammonium citrate, NaSb tartrate, KSb tartrate, NaW citrate, Cu ammonium citrate, Fe ammonium citrates, sodium metavanadate, V glycolate, V ammonium citrate, Zn ammonium citrate, Pd glycolate, Pd ammonium citrate and Sn glycolate.
 7. Process for colouring ceramic manufactured articles obtained by moulding a conventional ceramic mixture, said process using the composition as claimed in claim 1 and consisting in the following steps: a) drying the article to be coloured at 100° C. to a maximum water residue of 0.5% by wt.; b) treatment of the dried article with a water solution of the colouring composition in a quantity of 30 to 600 g/m2 of the final coloured surface; c) equalisation of the treated article at room temperature for 8 hours to homogenize the solution absorption; and d) oven firing according to the conventional ceramic cycle at a temperature of 1,000 to 1,300° C.
 8. The process as claimed in claim 5, wherein the solution of the colouring composition, thickened with thickening agents, is applied to the ceramic manufactured article in step (b) by a silk-screen technique.
 9. Vitrified stoneware tiles coloured at their surface and to a depth of at least 1 mm, in shades from pink to purple, to violet, by the process as claimed in any of claim 7 and
 8. 10. Vitrified stoneware tiles according to claim 9 wherein the surface layer has been removed by smoothing to a depth of 1.5 mm and final polishing.
 11. The process as claimed in claim 7, wherein the treatment with a gold thiolate solution is carried out so as to apply 0.1 to 20 g Au, as elemental Au, per m2 of surface to be coloured.
 12. Process for colouring ceramic manufactured articles obtained by moulding a conventional ceramic mixture, according to the process of claim 7 wherein a pre-treatment step is carried out on the dried article after step a) and before step b) said pre-treatment step comprises applying up to a maximum quantity of 300g/m₂ of water to the ceramic manufactured article.
 13. Process for colouring ceramic manufactured articles obtained by moulding a conventional ceramic mixture, according to claim 7 wherein a post-treatment step is carried out after step b) and before step c) said post-treatment step comprises applying up to a maximum of 300g/m₂ of water to the ceramic manufactured article. 